Field of the Invention
The present invention relates to a mobile communication system, a handover control method, a radio base station, and a mobile station. More particularly, the present invention relates to a mobile communication system in which a mobile station measures radio quality information between the mobile station and radio base stations and a radio base station as the source of handover decides the handover of the mobile station to a radio base station as the target of handover based on the result of the measurement so as to carry out a handover.
Description of the Related Art
The Super 3G (LTE or Long Term Evolution) system is considering transfer of data from a Source eNodeB which is a radio base station as the handover source to a Target eNodeB which is a radio base station as the handover target to realize a handover without data loss when a serving cell changes before and after a handover.
FIG. 12 is a sequence diagram illustrating the procedure of a handover of a related art (see 3GPP R3-060012, p. 6, “FIG. 6: Inter pool area HO sequence 1 (Active mode) for data communication terminal”, 10-12, Jan. 2006, for instance). In FIG. 12, a UE as a mobile station periodically measures radio quality between the UE and neighboring eNodeBs, and reports the result of the measurement to the Source eNodeB as a Measurement Report (step S301).
Based on the Measurement Report from the UE, the Source eNodeB determines whether to carry out a handover or not (step S302). When a handover is to be carried out, the Source eNodeB notifies the Target eNodeB of parameters for use at the Target eNodeB, such as UE information managed at the Source eNodeB, that is, context information necessary for communication with the UE, and gives a trigger to reserve radio resource of the Target eNodeB (step S303).
The Target eNodeB determines whether there is available radio resource or not (step S304), and if radio resource can be allocated to the UE, it notifies the Source eNodeB of completion of radio resource reservation (step S305). After radio resource of the Target eNodeB is reserved, the Source eNodeB notifies the UE of the parameters for use at the handover target (step S306). After the UE receives the parameters for use at the handover target, the UE and the Target eNodeB start operations for achieving synchronization (step S307).
After achieving synchronization, the UE notifies the Target eNodeB of completion of handover processing (step S308). After the handover processing, the Target eNodeB uses predefined radio resource to allocate a shared channel to the UE (step S309).
International Patent Publication No. WO92/01339 describes that a mobile station measures the strength of signals from neighboring base stations, and a base station as the source of handoff transfers measurement information pertaining to a base station as the target of handoff to the target base station when the mobile station is handed off to the target base station. This transfer is performed for the purpose of enabling a handoff with no waiting time when the mobile station is handed off from the target base station to other cell immediately after a handoff. That is to say, the target base station utilizes the transferred information for determining whether a handoff is necessary or not immediately after a handoff.
However, the sequence of a handover shown in FIG. 12 has such problems as follows.
A first problem is that optimal radio resource is likely not to be selected for a radio quality between the Target eNodeB and the UE for a while after the completion of a handover because the Target eNodeB does not have radio quality information between the Target eNodeB and the UE in relation to allocation of a shared channel to the UE by the Target eNodeB.
A second problem is possible occurrence of a handover sequence to other cell including the Source eNodeB immediately after a handover, as illustrated in FIG. 13. This is because, due to lack of radio quality information between the Target eNodeB and the UE at the Target eNodeB, the radio quality between the Target eNodeB and the UE does not meet conditions for accepting a handover in terms of the algorithm of the Target eNodeB, especially when the Source eNodeB and the Target eNodeB adopt different algorithms for handover decision (e.g., when the UE is handed over between eNodeBs from different vendors or operators). This phenomenon is also called a ping-pong phenomenon.
FIG. 13 is a sequence diagram illustrating the ping-pong phenomenon. As steps S401 through S408 of FIG. 13 are similar to steps S301 through S308 of FIG. 12, description of them is omitted.
In FIG. 13, after the handover of the UE to the Target eNodeB is completed (step S408), the Target eNodeB determines whether to carry out a handover to other cell or not based on a Measurement Report from the UE (step S409) as well as an algorithm of the Target eNodeB for handover decision. Here, if the radio quality between the Target eNodeB and the UE does not satisfy conditions for accepting a handover on the Target eNodeB, the Target eNodeB decides a handover to other cell immediately after the handover (step S410). As a result, a handover to other cell (in FIG. 13, the Source eNodeB) is performed again immediately after a handover is completed (steps S411 through S416). This is the ping-pong phenomenon. Steps S411 through S416 correspond to steps S403 through S408.